summation of n(21 - n) from n = 1 to n = 20
= summation of 21n - n^2 from n = 1 to n = 20
= [summation of 21n from n = 1 to n = 20] - [summation of n^2 from n = 1 to n = 20]
= [21(1 + 20)(20 / 2)] - [(1/6)(20)(20 + 1)(2(20) + 1)]
= 4410 - 2870
= 1540